U.S. patent application number 12/256973 was filed with the patent office on 2009-05-14 for method and apparatus for pelletizing a polymer feed.
Invention is credited to Ramin Abhari, Patrick S. Byrne, David A. Campbell, David R. Johnsrud, Jay L. Reimers.
Application Number | 20090121372 12/256973 |
Document ID | / |
Family ID | 40263404 |
Filed Date | 2009-05-14 |
United States Patent
Application |
20090121372 |
Kind Code |
A1 |
Campbell; David A. ; et
al. |
May 14, 2009 |
Method and Apparatus for Pelletizing A Polymer Feed
Abstract
A method and apparatus are described in which a polymer feed is
pelletized by introducing the polymer feed to an extruder, removing
heat from the polymer feed in the extruder, and extruding the
polymer feed through a pelletizing die.
Inventors: |
Campbell; David A.; (Crosby,
TX) ; Byrne; Patrick S.; (Baton Rouge, LA) ;
Abhari; Ramin; (Bixby, OK) ; Johnsrud; David R.;
(Humble, TX) ; Reimers; Jay L.; (Geismer,
LA) |
Correspondence
Address: |
EXXONMOBIL CHEMICAL COMPANY
5200 BAYWAY DRIVE, P.O. BOX 2149
BAYTOWN
TX
77522-2149
US
|
Family ID: |
40263404 |
Appl. No.: |
12/256973 |
Filed: |
October 23, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60986515 |
Nov 8, 2007 |
|
|
|
Current U.S.
Class: |
264/5 ;
425/6 |
Current CPC
Class: |
F26B 5/08 20130101; B29B
2009/165 20130101; B29B 9/065 20130101; F26B 17/00 20130101; F26B
3/08 20130101; B29B 9/16 20130101; B29K 2105/0097 20130101; B29B
9/12 20130101 |
Class at
Publication: |
264/5 ;
425/6 |
International
Class: |
B29B 9/12 20060101
B29B009/12; B28B 17/00 20060101 B28B017/00 |
Claims
1. A method for pelletizing a molten polymer feed having a
viscosity of less than 35,000 cP at 190.degree. C., comprising: (a)
introducing the molten polymer feed into an extruder; (b) cooling
the molten polymer feed while in the extruder to raise the
viscosity of the polymer feed to at least 5000 cP; and (c)
extruding the cooled polymer feed through a pelletizing die.
2. The method of claim 1 wherein the polymer feed is substantially
free of blowing agents.
3. The method of claim 1 wherein the polymer feed comprises at
least one propylene component.
4. The method of claim 1 wherein the polymer feed is substantially
free of styrene.
5. The method of claim 1 wherein the polymer feed comprises at
least 50 mol % of one or more C.sub.3 to C.sub.40 olefins where the
polymer has (a) a Dot T-Peel of 1 Newton or more on Kraft paper;
(b) a Mw of 10,000 to 100,000; (c) a branching index (g') of from
0.4 to 0.98 measured at the Mz of the polymer when the polymer has
a Mw of 10,000 to 70,000, or a branching index (g') of from 0.4 to
0.95 measured at the Mz of the polymer when the polymer has a Mw of
10,000 to 100,000; (d) a heat of fusion of 1 to 70 J/g; and (e) a
heptane insoluble fraction of 70 weight % or less, based upon the
weight of the polymer, where the heptane insoluble fraction has
branching index (g') of 0.9 or less as measured at the Mz of the
polymer.
6. The method of claim 1 wherein the polymer feed comprises at
least one additive chosen from the group comprising an antioxidant,
tackifier, wax, oil, or plasticizer.
7. The method of claim 6 wherein the additive comprises about 50%
or less of the total weight of the polymer feed.
8. The method of claim 1 wherein the extruder comprises a single
screw or a double screw.
9. The method of claim 1 wherein the polymer feed has a heat of
fusion (AH) less than 90 J/g.
10. The method of claim 1 wherein at least one component of the
polymer feed has a Mw of less than 70,000.
11. The method of claim 1 wherein the extruder generates at least
250 psi of driving force.
12. The method of claim 1 wherein the extruder is operating at an
outlet temperature less than the ring and ball softening
temperature of the polymer feed and greater than the
crystallization temperature of the polymer feed.
13. An apparatus for pelletizing a polymer feed, comprising: (a) an
extruder comprising an inlet, a barrel, and an outlet; (b) a heat
removing device adapted to remove heat from the barrel of the
extruder; and (c) an underwater pelletizing die attached to the
outlet of the extruder; wherein the polymer feed flows through the
extruder barrel and the heat removing device removes heat from the
polymer feed to raise the viscosity of the polymer feed to at least
5000 cP.
14. The apparatus of claim 13 further comprising a centrifugal
dryer, wherein the centrifugal dryer is attached to the outlet of
the underwater pelletizing die.
15. The apparatus of claim 13 wherein the extruder is a co-rotating
twin screw extruder.
16. The apparatus of claim 13 wherein the extruder generates at
least 250 psi of driving force.
17. The apparatus of claim 13 wherein the extruder is operating at
an outlet temperature less than the ring and ball softening point
but equal to or above the crystallization temperature of the
polymer feed.
18. A method for pelletizing a molten polymer feed, having a
viscosity at 190.degree. C. of less than 35,000 cP, the method
comprising: (a) introducing the molten polymer feed into an
extruder; (b) cooling the molten polymer feed while in the extruder
to raise the viscosity of the polymer feed to greater than 5000 cP;
and (c) extruding the cooled polymer feed through a pelletizing
die; wherein the polymer feed is cooled to a temperature less than
the ring and ball softening point of the polymer feed but greater
than the crystallization temperature of the polymer feed while in
the extruder.
19. The method of claim 18, wherein the extruder generates at least
250 psi of driving force.
20. A method for pelletizing a molten polymer feed having a
viscosity of less than 35,000 cP at 190.degree. C., comprising: (a)
introducing the molten polymer feed into an extruder; (b) cooling
the molten polymer feed while in the extruder to raise the
viscosity of the polymer feed to at least 5000 cP; and (c)
extruding the cooled polymer feed through a pelletizing die;
wherein the polymer feed is cooled to a temperature near the
crystallization temperature of the polymer feed while in the
extruder and the extruder increases the dispersive homogeneity of
the polymer melt.
Description
PRIORITY CLAIM
[0001] This application claims priority to and the benefit of U.S.
Ser. No. 60/986,515, filed Nov. 08, 2007 which is hereby
incorporated by reference.
FIELD
[0002] This invention relates to a method and apparatus for
pelletizing a polymer feed, such as a hot melt adhesive polymer
feed.
BACKGROUND
[0003] Olefin based polymers are widely used in various
applications due to their being chemically inert, having low
density, and low cost. Applications include adhesives, films,
fibers, molded parts, and combinations thereof. While these
polymers are solid at room temperature, they are often produced and
processed as melts. The last step in the manufacturing process for
such materials is converting the polymer melt into easily handled
granules. Granules--pellets being one type--are advantageous as
they can be easily packaged, transported, weighed/batched, and
reprocessed.
[0004] Over the years, two different classes of granulation
technology have evolved: granulation technology for low viscosity
melts, e.g., viscosity less than 100 cP, and granulation technology
for high viscosity melts, e.g. viscosity greater than 100,000 cP.
Granulation of low viscosity melts is generally characterized by
(1) applying a low viscosity melt onto a cooling surface, (2)
cooling the melt into a solid, and (3) recovering the solid as
flakes, pastilles, briquettes, granules, or other suitable forms.
Often, however, the granulation step is skipped altogether for low
viscosity melts, and the melts are packaged in transportable melt
tanks. Granulation of high viscosity melts generally involves (1)
extruding the high viscosity melt through a die and (2) cooling and
cutting the resulting strands into pellets.
[0005] Although techniques have been developed for granulation of
low viscosity and high viscosity melts, there is a gap in
granulation technology for materials of intermediate viscosity,
such as hot melt adhesives (HMAs). In general, melts with an
intermediate viscosity have lower melt strength than melts with a
high viscosity. This lower melt strength translates into a polymer
melt that cannot be easily cut with traditional pelletizing
techniques as the polymer melt has little to no definition or form.
Thus, when these traditional granulation techniques are attempted
with polymer melts having intermediate viscosity polymer wrap-ups
around the cutter assembly often result.
[0006] Additionally, regardless of how the granules are formed, the
tackiness of HMAs is a factor affecting HMA granulation. If the
surface of the granule is tacky, this can lead to granule
agglomeration. Agglomerated granules are then more difficult to
re-melt for subsequent use than free-flowing, non-agglomerated,
granules.
[0007] Conventional processes have attempted to address some of the
problems associated with pelletizing HMAs, such as cutter wrap-ups
and granule agglomeration, however, these and other problems still
exist. For example, another problem associated with HMA
pelletization is that as the polymer melt is cooled,
crystallization often begins to occur and the polymer melt may lose
homogeneity as the polymer components in the melt disperse and fall
out of solution.
[0008] Furthermore, pelletization can also be difficult when the
material being pelletized exhibits: a wide melting range, multiple
melting ranges, a low temperature melting range, an intermediate
viscosity, slow thermal conductivity and thus a lesser ability to
cool rapidly for processing, a proclivity to undergo phase
separation on cooling, delayed crystallization, surface tack,
and/or an extreme temperature variance from the mixing and blending
stage to the extrusion and pelletization stage, as all of these
qualities can lead to poor pellet formation and poor pellet
geometry.
[0009] Accordingly, there remains a need for a method and apparatus
to pelletize polymer melts with an intermediate viscosity and/or
polymer melts which exhibit delayed crystallization. In particular,
it would be desirable to have a method to pelletize HMA
compositions.
SUMMARY
[0010] Provided are methods of pelletizing a polymer feed composed
of the steps of introducing a polymer feed into an extruder,
cooling the molten polymer feed while in the extruder to increase
the viscosity of the polymer feed, and extruding the cooled polymer
feed through a pelletizing die.
[0011] For example, methods of pelletizing a polymer feed having a
viscosity at 190.degree. C. of from about 10 cP to about 75,000 cP
or from 100 cP to about 35,000 include the steps of introducing a
molten polymer feed into an extruder, cooling the polymer feed
while in the extruder to a pelletization temperature to raise the
viscosity of the polymer feed to greater than 5000 cP, and
extruding the cooled polymer feed through a pelletizing die. The
pelletizing temperature may be: (a) sufficiently near, but above,
the ring and ball softening point of the polymer feed while in the
extruder such that the extruder increases the dispersive
homogeneity of the polymer melt, (b) less than the ring and ball
softening point of the polymer feed, (c) less than the ring and
ball softening point of the polymer feed but greater than the
crystallization temperature of the polymer feed while in the
extruder, (d) sufficiently near, but above, the crystallization
temperature of the polymer feed while in the extruder such that the
extruder increases the dispersive homogeneity of the polymer melt,
or (e) at or below the crystallization temperature of the polymer
feed. In one embodiment, the extruder creates a pressure at the die
face of at least 250 psi to force the cooled polymer feed through a
pelletizing die.
[0012] Also provided is an apparatus for pelletizing a polymer feed
composed of a melt cooler with an inlet and an outlet; an extruder
with an inlet, a barrel, and an outlet, wherein the extruder inlet
is attached to the melt cooler outlet; a heat removing device
adapted to remove heat from the barrel of the extruder; and a
pelletizing die attached to the outlet of the extruder e.g., an
underwater pelletizing device; wherein the polymer feed flows
through the extruder barrel and the heat removing device removes
heat from the polymer feed to raise the viscosity of the polymer
feed.
[0013] In a further embodiment, an apparatus for pelletizing a
polymer feed comprises a melt cooler with an inlet and an outlet;
an extruder with an inlet, a barrel, and an outlet, wherein the
extruder inlet is attached to the melt cooler outlet; a heat
removing device adapted to remove heat from the barrel of the
extruder; and an underwater pelletizing die attached to the outlet
of the extruder; wherein the polymer feed flows through the
extruder barrel and the heat removing device removes heat from the
polymer feed to increase the viscosity of the polymer feed to at
least 5,000 cP.
BRIEF DESCRIPTION OF THE FIGURES
[0014] FIG. 1 is a schematic illustration of an apparatus for
pelletizing a polymer feed composed of a melt cooler, an underwater
pelletizer, and a drying apparatus.
[0015] FIG. 2 is a schematic illustration of an apparatus for
pelletizing a polymer feed composed of a melt cooler, a cooling
extruder, an underwater pelletizer, and a drying apparatus.
[0016] FIG. 3 is a schematic illustration of an apparatus for
pelletizing a polymer feed composed of an extruder, an underwater
pelletizer, and a drying apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] Provided are methods and apparatus for pelletizing a polymer
feed. The methods are composed of the steps of introducing a
polymer feed into an extruder, cooling the molten polymer feed
while in the extruder to raise the viscosity of the polymer feed,
and extruding the cooled polymer feed through a pelletizing die.
The polymer feed is cooled to a pelletizing temperature that may
be: (a) sufficiently near, but above, the ring and ball softening
point of the polymer feed while in the extruder such that the
extruder increases the dispersive homogeneity of the polymer melt,
(b) less than the ring and ball softening point of the polymer
feed, (c) less than the ring and ball softening point of the
polymer feed but greater than the crystallization temperature of
the polymer feed while in the extruder, (d) sufficiently near, but
above, the crystallization temperature of the polymer feed while in
the extruder such that the extruder increases the dispersive
homogeneity of the polymer melt, or (e) at or below the
crystallization temperature of the polymer feed. In one embodiment,
the extruder creates a pressure at the die face of at least 250 psi
to force the cooled polymer feed through a pelletizing die.
[0018] The methods and apparatus described herein are useful for
pelletizing polymer feeds which are not easily pelletized,
particularly those polymer feeds with an intermediate viscosity,
polymers exhibiting a delayed crystallization, polymers that
exhibit a wide melting range, have multiple melting ranges, or have
a low temperature melting range. The method and apparatus are also
particularly suitable for pelletizing polymer feeds which exhibit
(a) sharp viscosity increases and undergo fouling or phase
separation when cooled, (b) slow thermal conductivity and thus a
lesser ability to cool rapidly for processing, (d) surface tack, or
(d) a high temperature variance from the mixing and blending stage
to the extrusion and pelletization stage. Each of these so called
difficult-to-process qualities have led to poor pellet formation
and poor pellet geometry in conventional pelletizing processes.
Beneficially, the undesirable effects of these qualities are
reduced by the methods and apparatus described herein.
[0019] In particular, the provided method and apparatus are useful
for pelletizing an HMA polymer feed, wherein the HMA polymer feed
is cooled to a temperature so that the viscosity is raised to a
level where the HMA polymer feed is readily pelletized, all the
while mixing the polymer feed to increase the dispersive
homogeneity of the polymer feed.
[0020] Various specific embodiments, versions, and examples are
described herein, including exemplary embodiments and definitions
that are adopted for purposes of understanding the claimed
invention. While the following detailed description gives specific
preferred embodiments, those skilled in the art will appreciate
that these embodiments are exemplary only, and that the invention
can be practiced in other ways. For purposes of determining
infringement, the scope of the invention will refer to any one or
more of the appended claims, including their equivalents, and
elements or limitations that are equivalent to those that are
recited. Any reference to the "invention" may refer to one or more,
but not necessarily all, of the inventions defined by the
claims.
Polymer Feed
[0021] The polymer feed is composed of polymers that include
C.sub.2 to C.sub.40 olefins and blends thereof. Preferably the
olefin is a homo or copolymer of propylene. In an embodiment, the
polymer feed comprises at least one propylene component. In a
preferred embodiment the polymer comprises at least 50 wt %
propylene, preferably at least 60 wt % propylene, alternatively at
least 70 wt % propylene, alternatively at least 80 wt % propylene
based on the total weight of the polymer.
[0022] In still another embodiment of the invention, the polymer
feed is substantially free of styrene. In an embodiment, the
polymer feed has 5 wt % or less of styrene, or preferably 3 wt % or
less of styrene, or more preferably 1 wt % or less of styrene based
on the total weight of the polymer.
[0023] In a preferred embodiment of the invention, the polymer feed
is molten. A molten polymer feed is polymer feed which is in a form
capable of being extruded. A molten polymer feed can be in melt
form, semi-solid form, substantially liquid form, or liquid form. A
molten feed may flow suitably by gravity or under pressure when
released in batch processing or continuous flow processing.
Preferably, the polymer feed is in a molten form prior to being
extruded, and the extruder is not being used to melt the polymer
feed.
[0024] In a preferred embodiment, the polymer feed is one which is
prone to undergo phase separation upon cooling. Preferably, the
polymer feed is composed of two or more components, one or more of
which undergo crystallization as the polymer cools. As the
component crystallizes the polymer melt may lose homogeneity as the
polymer component in the melt disperses and falls out of solution
forming a phase separation. In one embodiment, the polymer feed
comprises an isotactic-polypropylene component which crystallizes
as the polymer feed is cooled. The crystallized
isotactic-polypropylene component may form a highly viscous layer
on the surface of the melt cooler (heat exchanger), impeding
further heat transfer as the polymer feed is cooled.
[0025] In some embodiments, the polymer feed exhibits delayed
crystallization during cooling, concurrent with pelletizing, or
subsequent to pelletizing, e.g., during storage. Described more
particularly, as the polymer feed is cooled, crystallization begins
to form. However, without being limited by theory, it is believed
that the rate that the polymer feed is cooled effects the rate of
crystallization such that the faster the polymer feed is cooled the
longer it will take to fully crystallize. This, so called, delayed
crystallization phenomena may be seen throughout the polymer feed,
throughout the polymer pellets, or as fractionated crystallization
in only regions of the polymer feed or pellets. For example, the
exterior of individual pellets may be cooled more rapidly than the
pellet interior thereby resulting in fractionated crystallization
on the surface of each pellet. On a preferred embodiment, the
polymer feed is cooled very rapidly, i.e., quench cooling, in an
underwater pelletizer or in a melt cooler. In such embodiments,
fractionated crystallization is more readily observed, e.g., at the
outer portion of the polymer feed along the melt cooler's walls.
Thus, the entirety of the polymer feed may not cool at the same
rate. The provided methods and apparatus reduce the detrimental
effects of utilizing and pelletizing such difficult-to-pelletize
materials.
[0026] In an embodiment, the polymer feed exhibits a sharp change
in viscosity as the polymer feed approaches the polymer feed's
crystallization temperature.
[0027] In an embodiment of the invention, the polymer feed has a
measurable melt point and a ring and ball softening point as
measured according to ASTM D6493. In a preferred embodiment, the
ring and ball softening point of the polymer feed is greater than
the crystallization point of the polymer feed as measured by ASTM E
794-06. Preferably, the ring and ball softening point is 10.degree.
C. or more above the crystallization temperature, or more
preferably 20.degree. C. or more above the crystallization
temperature.
[0028] In another embodiment of the invention, the polymer feed
comprises an amorphous polymer. The polymer feed can have an
amorphous content of at least 50%, alternatively at least 60%,
alternatively at least 70%, even alternatively between 50 and 99%.
The percent of amorphous content is determined using Differential
Scanning Calorimetry measurement according to ASTM E 794-06.
[0029] In yet another embodiment of the invention, the polymer feed
comprises a polymer with a crystallinity of 50% or less,
alternatively 40% or less, alternatively 30% or less, alternatively
20% or less, even alternatively between 10% and 30%. Percent
crystallinity content is determined using Differential Scanning
Calorimetry measurement according to ASTM E 794-06. In another
embodiment, the polymer feed comprises a polymer with a percent
crystallinity of between 5% and 60%, alternatively between 10% to
50%.
[0030] In an embodiment of the invention, the polymer feed has a
heat of fusion (AH) of 100 J/g or less, preferably 90 J/g or less,
or 70 J/g or less, or 60 J/g or less, or 50 J/g or less, or 40 J/g
or less, or 30 J/g or less, or 20 J/g or less and greater than
zero, or greater than 1 J/g, or greater than 10 J/g, or between 10
and 50 J/g. Heat of fusion is measured according to ASTM E
794-06.
[0031] In an embodiment of the invention, the polymer feed has a
viscosity (also referred to as a Brookfield Viscosity or Melt
Viscosity) at 190.degree. C. of less than about 100,000 cP, but may
be higher for some compositions. Preferably, polymer feeds have a
viscosity at 190.degree. C. of less than about 50,000 cP, or less
than about 35,000 cP, or 30,000 cP, or less at 190.degree. C.; or
25,000 cP or less at 190.degree. C.; or 20,000 cP or less at
190.degree. C.; or 15,000 cP or less at 190.degree. C.; or 10,000
cP or less at 190.degree. C.; or 8,000 cP or less at 190.degree.
C.; or 6,000 cP or less at 190.degree. C.; or 5,000 cP or less at
190.degree. C.; or 4,000 cP or less at 190.degree. C.; or 3,000 cP
or less at 190.degree. C.; or 2,000 cP or less at 190.degree. C.;
or 1,000 cP or less at 190.degree. C. as measured by ASTM D 3236 at
190.degree. C. In another embodiment, the polymer feed has a
viscosity in the range of from about 100 cP at 190.degree. C. to
about 35,000 cP at 190.degree. C. In a further embodiment of the
invention, the polymer feed has a viscosity of less than 35,000 cP
at the polymer feed's process conditions; or 20,000 cP or less; or
15,000 cP or less; or 10,000 cP or less; or 5,000 cP or less; or
3,000 cP or less; or 2,000 cP or less; or 1,000 cP or less; or 900
cP or less; or 800 cP or less; or 700 cP or less; or 600 cP or
less; 500 cP or less; or alternatively, from 100 cP to 35,000 cP,
or from 500 cP to 20,000 cP; or from 800 cP to 15,000 cP.
Preferably these viscosities are the viscosity of the polymer feed
prior to entering either the melt cooler or the cooling
extruder.
[0032] In an embodiment of the invention at least one component of
the polymer feed has a weight average molecular weight (Mw) of less
than 70,000 or less, alternately about 60,000 or less, alternately
about 50,000 or less, or alternately about 40,000 or less.
Alternately, at least one component of the polymer feed has a Mw in
the range of from about 10,000 to about 70,000. The molecular
weight is measured by using a Waters 150 SizeExclusion
Chromatograph (SEC) equipped with a differential refractive index
detector (DRI), an online low angle light scattering (LALLS)
detector and a viscometer (VIS).
[0033] In a further embodiment of the invention, the polymer feed
is substantially free of blowing agents. Substantially free of
blowing agents is defined to mean that the polymer feed is largely,
but not wholly, absent blowing agents. In some embodiments, small
amounts of blowing agents may be present within the polymer feed as
a result of standard manufacturing methods. In one embodiment
"substantially free of blowing agents" means free of intentionally
added blowing agents, in another embodiment it means free of any
blowing agents. Blowing agents are generally either chemical
blowing agents or physical blowing agents. Generally, chemical
blowing agents undergo some form of chemical change (e.g., a
chemical reaction with the polymer material at a predetermined
temperature/pressure) that causes the release of a gas, such as
nitrogen, carbon dioxide, or carbon monoxide. Generally, physical
blowing agents are dissolved in the polymer material under pressure
and then expand volumetrically when the pressure is removed.
Blowing agents can include halocarbons, hydrocarbons, atmospheric
gases, and combinations thereof. Non-limiting examples of blowing
agents include dichlorodifluromethane (CFC-12);
trichlorofluromethane (CFC-11); C.sub.2-C.sub.6 alkanes such as
ethane, propane, butane, isobutane, pentane, isopentane, and
hexane; carbon dioxide; argon; and nitrogen.
[0034] In another embodiment, the polymer feed is substantially
free of gases. Substantially free of gases is defined to mean that
the polymer feed is largely, but not wholly, absent gases. In some
embodiments, small amounts of gases may be present within the
polymer feed as a result of standard manufacturing methods. In one
embodiment "substantially free of gas" means free of intentionally
added gases, in another embodiment it means free of any gas. Gases
include, but are not limited to, blowing agents, carbon dioxide
(CO.sub.2), and nitrogen (N.sub.2).
[0035] In a preferred embodiment of the invention the polymer feed
comprises a hot melt adhesive (HMA). Preferably the HMA is a
polyolefin adhesive. Polyolefin adhesive compositions to be
utilized in this invention may be, for example, the polyolefin
adhesive compositions disclosed in U.S. Pat. No. 7,223,822 B1, U.S.
Patent Application Pub. No. 2004/0127614 A1, U.S. Patent
Application Pub. No. 2004/0138392 A1, U.S. Patent Application Pub.
Nos. 2004/0220320 A1, 2004/0220336 A1, and 2004/0249046 A1, all
incorporated herein by reference. Conventional methods and
apparatus for preparing olefin compositions are disclosed in U.S.
Pat. Nos. 4,054,632, 5,041,251, 5,403,528, 6,238,732 B1, 6,894,109
B1, EP Publication No. 0 410 914 B1, and PCT Publication No. WO
2007/064580 A2, each of which is herein incorporated by reference
in its entirety.
[0036] In a preferred embodiment, the polymer feed comprises at
least 50 mol % of one or more C.sub.3 to C.sub.40 olefins where the
polymer has a Dot T-Peel of 1 Newton or more on Kraft paper; a Mw
of 10,000 to 100,000; a branching index (g') of from 0.4 to 0.98
measured at the Mz of the polymer when the polymer has a Mw of
10,000 to 70,000, or a branching index (g') of from 0.4 to 0.95
measured at the Mz of the polymer when the polymer has a Mw of
10,000 to 100,000; a heat of fusion of 1 to 70 J/g; and a heptane
insoluble fraction of 70 weight %or less, based upon the weight of
the polymer, where the heptane insoluble fraction has branching
index g' of 0.9 or less as measured at the Mz of the polymer. The
Mw and the z-average molecular weight (Mz) can be determined by
using a Waters 150 SizeExclusion Chromatograph (SEC) equipped with
a differential refractive index detector (DRI), an online low angle
light scattering (LALLS) detector and a viscometer (VIS). The
branching index (g') is measured using SEC with an on-line
viscometer (SEC-VIS) and is reported as g' at each molecular weight
in the SEC trace. The branching index g' is defined as:
g'=.eta..sub.b/.eta..sub.1, where .eta..sub.b is the intrinsic
viscosity of the branched polymer and .eta..sub.1 is the intrinsic
viscosity of a linear polymer of the same viscosity-averaged
molecular weight as the branched polymer. As used herein, Dot
T-Peel is determined according to ASTM D 1876, except that the
specimen is produced by combining two 1 inch by 3 inch (2.54
cm.times.7.62 cm) Kraft paper substrate cut outs with a dot of
adhesive with a volume that, when compressed under a 500 gram
weight occupies about 1 square inch of area (1 inch=2.54 cm). Once
made, all the specimens are pulled apart in side by side testing
(at the rate of 2 inches per minute) by a device which records the
destructive force being applied. The maximum force achieved for
each sample tested was recorded and averaged, thus producing the
average maximum force, which is reported as the Dot T-Peel.
[0037] In one embodiment of the invention, the polymer feed
comprises at least one additive. The additive may comprise about
50% or less by weight of the total weight of the feed, or 40% or
less by weight of the total weight of the feed, or 30% or less by
weight of the total weight of the feed, or 20% or less by weight of
the total weight of the feed, or 10% or less by weight of the total
weight of the feed.
[0038] Additives useful in embodiments of this invention may be
solid or liquid. The additives may be in the molten polymer feed
before the polymer feed enters the extruder, or alternatively the
additives may be added into the polymer feed as side injections
into the extruder. Additives can be melted in a side-arm extruder,
and then blended into the polymer. In one embodiment there may be
one or more feeding and injection ports along the barrel of the
extruder to allow for the addition of additives to the molten
polymer feed. More than one additive may be incorporated into the
polymer feed.
[0039] Useful additives can be chosen from the group consisting of:
another polymer, fillers, antioxidants, adjuvants, adhesion
promoters, tackifiers, waxes, oils, plasticizers, or the like, or
mixtures thereof. Preferred additives include silicon dioxide,
titanium dioxide, polydimethylsiloxane, talc, dyes, wax, calcium
state, carbon black, low molecular weight resins, and glass beads.
Other preferred additives include block, antiblock, pigments,
processing aids, UV stabilizers, hindered amine light stabilizers,
UV absorbers, neutralizers, lubricants, surfactants, and nucleating
agents. Preferred fillers include, but are not limited to, titanium
dioxide, calcium carbonate, barium sulfate, silica, silicon
dioxide, carbon black, sand, glass beads, mineral aggregates, talc,
clay, and the like. Preferred adhesion promoters include polar
acids, polyaminoamides, urethanes, coupling agents, titanate
esters, reactive acrylate monomers, metal acid salts, polyphenylene
oxide, oxidized polyolefins, acid modified polyolefins, and
preferably anhydride modified polyolefins. Preferred plasticizers
include mineral oils, polybutenes, phthalates, and the like.
Particularly preferred oils include aliphatic napthenic oils.
Preferred waxes may include both polar and non-polar waxes,
functionalized waxes, polypropylene waxes, polyethylene waxes, and
wax modifiers.
Cooling Extruder
[0040] In an embodiment of the invention, a cooling extruder is
used to cool the polymer feed. As the polymer melt is cooled along
the length of the extruder, the effective viscosity of the polymer
melt increases as the melt's temperature is lowered. In a preferred
embodiment the polymer feed is cooled down in order to raise the
viscosity of the polymer feed to at least about 5,000 cP for
pelletizing.
[0041] In a particularly preferred embodiment, a cooling extruder
is used to provide efficient mixing of the polymer feed while at
the same time providing controlled cooling of the molten material.
The cooling extruder provides for precise control of the polymer
melt's temperature as the melt arrives at the pelletizing die face.
The cooling extruder also provides a way to homogenize and
accurately control the temperature of the polymer feed so that
homogeneous and uniform pellets of any size may be made.
Preferably, the cooling extruder provides dispersive mixing of the
polymer feed to eliminate any phase separation of the blended
components of the polymer feed.
[0042] The cooling extruder comprises an inlet, a barrel, and an
outlet. The inlet is where the polymer feed is introduced into the
extruder. The polymer feed then travels down the extruder barrel,
and out the extruder outlet.
[0043] In an embodiment of the invention, the extruder comprises a
single screw. In another embodiment, the extruder comprises a
double screw. In a further embodiment of the invention, the
extruder is a co-rotating twin screw extruder. In an embodiment of
the invention, the extruder has three or more screws.
Alternatively, the extruder can have a ring design.
[0044] In another embodiment of the invention the extruder
comprises at least one screw with continuous flights. In yet
another embodiment of the invention, the extruder comprises at
least one screw with discontinuous flights.
[0045] In one embodiment, a useful extruder may have a cooling
barrel comprising a wall, at least one central shaft having a screw
with flights, a screw speed (.nu.), a pitch angle (.theta.), a
flight width (w), a screw height (h), an inner barrel diameter (D),
a barrel length (L), a molten polymer feed rate (FR), and a
clearance distance between the flight and the cooling barrel
(.delta.).
[0046] In one embodiment the extruder has a clearance between the
flight and the cooling barrel (.delta.) of about 0.0005 m to about
0.005 m, preferably the clearance is about 0.001 m. In an
embodiment the extruder has a screw height (h) of about 0.004 m to
about 0.02 m, preferably the screw height is about 0.01 m. In an
embodiment the extruder has a pitch angle (.theta.) of about
40.degree. to about 50.degree., preferably the pitch angle is about
45.degree.. In an embodiment the extruder has a flight width (w) of
about 0.1 m to about 0.3 m, preferably the flight width is about
0.2 m. In an embodiment the extruder has a screw speed (.nu.) of
about 80 rpm to about 100 rpm, preferably the screw speed is about
90 rpm. As the screw speed increases the polymer feed may be
heated, thus increasing the temperature of the polymer feed. Thus,
it is preferred that the screw speed remain at such a speed so as
to not heat the polymer feed causing the viscosity of the polymer
feed to decreased so that the polymer feed can no longer be easily
pelletized but yet remain at a high enough speed sufficient to
develop the necessary pressure to drive the polymer feed through
the pelletizing die.
[0047] In one embodiment the extruder has an inner barrel diameter
(D) of about 80 mm to about 100 mm, preferably the barrel diameter
is about 90 mm. In an embodiment the extruder has a barrel length
(L) of about 5 m to about 6 m, preferably the length is about 5.5
m. In an embodiment the extruder has a length to diameter ratio
(L/D) of about 50 to about 80, preferably the length to diameter
ratio is about 60. In general, the cooling becomes less efficient
as the melt progresses along the extruder, thus increasing the
length of the extruder may not necessarily improve cooling
capacity.
[0048] In one embodiment the feed rate (FR) of the polymer feed
into the cooling extruder is from about 500 lb/hr to about 40,000
lb/hr, or from about 1000 lb/hr to about 30,000 lb/hr, or from
about 2000 lb/hr to about 20,000 lb/hr. The feed rate of the
polymer feed may vary greatly depending on the size of the
apparatus being used. For example, for a pilot plant the feed rate
may be less than 500 lb/hr; for a small plant the feed rate may be
from about 1000 lb/hr to about 8000 lb/hr; for a large world scale
plant facility the feed rate may be greater than 10,000 lb/hr, or
even greater than 20,000 lb/hr.
[0049] Extruders useful in this invention include those
commercially available from MARIS S.p.A., Century, Inc. of Traverse
City, Mo., or Coperion Corporation of Ramsey, N.J., such as the
Coperion ZSK-25 twin-screw extruder.
[0050] The extruder used in this invention may be used to provide a
means for pressurizing and forwarding the polymer melt. In an
embodiment the screw within the extruder can have sections with
different numbers of flights. For example, the screw flights may be
more closely spaced together near the extruder outlet in order to
provide the desired pressure needed to force the polymer feed
through the pelletizing die.
[0051] In another embodiment, the extruder creates at least 250
psi, or at least 300 psi, or at least 500 psi, or at least 1000
psi, or at least 2000 psi, or at least 3000 psi, or at least 4000
psi, or at least 5000 psi of driving force to drive the polymer
feed through the pelletizing die. Alternatively, the extruder
creates from about 250 to about 1000 psi of driving force, or more
preferably from about 400 psi to about 1000 psi.
[0052] In an alternative embodiment, a melt pump may be used to
create an additional driving force to forward the polymer feed
through the pelletizing die. The melt pump may be located after the
extruder. Alternatively, the melt pump may be located before the
extruder. The melt pump may generate at least 200 psi of pressure
on the polymer feed, more preferably from about 500 psi to about
2000 psi. The melt pump may be centrifugal, positive displacement,
reciprocating, or rotary pump. Preferably, the melt pump is a
rotary pump which may be peristaltic, vane, screw, lobe, or
progressive cavity. Most preferably, the melt pump is a gear pump.
The melt pump may be used as a booster pump to build on the
pressure already created by the cooling extruder.
[0053] In one embodiment of the invention, the polymer feed is
cooled as it moves down the barrel of the extruder. The cooling
extruder provides a method for controlled cooling of the polymer
feed, while providing efficient mixing of the polymer feed. The
polymer may be cooled by a transfer of heat from the polymer feed
through the extruder wall into a cooling medium.
[0054] In one embodiment a conventional cooling extruder with
drilled cooling pathways may be used. A cooling medium may flow
through the drilled cooling pathways to remove heat from the
polymer feed. In another embodiment the extruder comprises a
cylindrical tube and a second larger diameter cylindrical tube
oriented coaxially to the extruder forming an outer cooling pathway
around the extruder.
[0055] A cooling medium, such as water or any other material having
a lower temperature than the feedstock in the extruder, may be used
to cool the polymer feed. The cooling medium may pass through the
cooling pathways and withdraw heat from the polymer feed. The
temperature of the polymer feed can be modified by varying the flow
rate and/or temperature of the cooling medium which passes through
the extruder's cooling pathway.
[0056] In another embodiment the cooling medium can be in the
extruder's screw shaft. An example of a useful extruder where the
cooling medium can be found in the screw shaft can be found in U.S.
Patent Application Publication No. 2005/0236734 A1, incorporated
herein by reference.
[0057] In an embodiment of the invention, water is used as a
cooling medium to remove heat from the barrel of the extruder. In
another embodiment of the invention, water and glycol are used as a
cooling medium to remove heat from the barrel of the extruder. In a
further embodiment of the invention, cold gases can be used as a
cooling medium to remove heat from the barrel of the extruder.
Useful cold gases are carbon dioxide and propane. The cooling
medium may be any medium which is a fluid suitable for heat
dissipation, such as water, salt solutions, brine, ethylene glycol
chilled water, or low-melting-point organic compounds.
[0058] In one embodiment the cooling medium temperature is about
50.degree. F. or less, or about 45.degree. F. or less, or about
40.degree. F. or less, or about 35.degree. F. or less. In another
embodiment the cooling water temperature is from about 50.degree.
F. to about 55.degree. F. Preferably the temperature of the cooling
water is about 55.degree. F.
[0059] In an embodiment the temperature of the polymer feed at the
cooling extruder inlet is from about 220.degree. F. to about
260.degree. F. In an embodiment the temperature of the polymer feed
at the cooling extruder outlet is from about 210.degree. F. to
about 230.degree. F. In one embodiment the polymer feed is cooled
so that the difference in the inlet temperature and the outlet
temperature is at least 5.degree. F., or at least 10.degree. F., or
at least 20.degree. F., or at least 30.degree. F., or at least
50.degree. F. In another embodiment, the temperature of the polymer
feed at the cooling extruder inlet is from about 160.degree. F. to
about 550.degree. F., alternatively from about 200.degree. F. to
about 400.degree. F., or from about 220.degree. F. to about
260.degree. F. The temperature of the polymer feed at the cooling
extruder outlet may be from about 75.degree. F. to about
400.degree. F., or from about 100.degree. F. to about 300.degree.
F., or from about 200.degree. F. to about 250.degree. F., or from
about 210.degree. F. to about 230.degree. F.
[0060] The extruder screw can be used to mix and homogenize the
polymer feed. The extruder may be used to mix and homogenize any
crystallization or solid precipitation that may form in the polymer
feed. The extruder can be used to enhance the dispersion of the
materials in the polymer feed, thus eliminating any phase
separation that may occur. The extruder can be used to keep the
polymer components in solution. In an embodiment, the blades on the
extruder screw may be used to wipe the walls of the extruder, thus
preventing any crystallization from forming on the extruder
walls.
[0061] In a preferred embodiment, the temperature of the polymer
feed at the extruder outlet is less than the ball-and-ring
softening temperature of the polymer feed yet greater than the
crystallization temperature of the polymer feed.
[0062] In an alternate embodiment, the extruder is operating at an
outlet temperature less than the crystallization temperature of the
polymer feed, but the viscosity of the polymer feed remains
sufficiently high to produce pellets. In an embodiment the extruder
is operating at an outlet temperature less than the crystallization
temperature of the polymer feed but the viscosity of the polymer
feed is at least 5000 cP at 190.degree. C. In an embodiment of the
invention, the extruder is operating at an outlet temperature less
than the crystallization temperature of the polymer feed. In an
embodiment the molten polymer feed is cooled to a temperature below
the crystallization temperature of the polymer feed. The outlet
temperature may be 1.degree. C. or more, 5.degree. C. or more, or
10.degree. C. or more, or 20.degree. C. or more lower than the
crystallization temperature of the polymer feed.
[0063] In a further embodiment, two or more cooling extruders may
be used in parallel to cool the polymer melt prior to extrusion. A
useful example of using two extruders in parallel can be found in
U.S. Patent Application Publication No. 2003/0094718 A1,
incorporated herein by reference. In another embodiment, two or
more cooling extruders may be used in series to cool the polymer
melt.
[0064] In an embodiment of the invention, the method and apparatus
further comprise the use of a heat exchanger. A heat exchanger,
e.g., melt cooler, can be used to cool the polymer feed before the
polymer feed enters the extruder. Alternatively, a heat exchange
can be used to further cool the polymer feed after the polymer feed
exits the extruder. The heat exchanger may be a melt cooler of the
coil type, scrape wall, plate and frame, shell or tube design with
or without static mixers. Preferably a shell and tube design melt
cooler which includes static mixing blades within the individual
tubes is used.
Underwater Pelletizer
[0065] The cooled extruded polymer feed is pelletized.
Pelletization of the polymer feed may be by an underwater, hot
face, strand, water ring, or other similar pelletizer. Preferably
an underwater pelletizer is used, but other equivalent pelletizing
units known to those skilled in the art may also be used. General
techniques for underwater pelletizing are known to those of
ordinary skill in the art. Examples of useful underwater
pelletizing devices can be found in U.S. Pat. Nos. 7,033,152 B2,
7,226,553 B2, and U.S. Patent Application Publication No.
2007/0119286 A1, all incorporated herein by reference.
[0066] In one embodiment an underwater pelletizer is used to
pelletize the cooled extruded polymer feed. The cooled polymer feed
is extruded through a pelletizing die to form strands. The strands
are then cut by rotating cutter blades in the water box of the
underwater pelletizer. Water continuously flows through the water
box to further cool and solidify the pellets and carry the pellets
out of the underwater pelletizer's water box for further
processing.
[0067] In one embodiment, the pelletizing die is thermally
regulated by means known to those skilled in the art in order to
prevent die hole freeze-off.
[0068] In an embodiment the underwater pelletizer uses chilled
water, thus allowing for further rapid cooling of the pellets and
solidification of the outermost layer of the pellets. In an
embodiment, the temperature of the water in the underwater
pelletizing unit may be from about 35.degree. F. to about
75.degree. F. Preferably a water chilling system is able to cool
the water going to the underwater pelletizer water box (cutting
chamber) down to about 40.degree. F.
[0069] In an embodiment, the underwater pelletizer unit has a
chilled water slurry circulation loop. The chilled water helps
eliminate the tendency of the pellets to stick together and allows
the extruded polymer strands to be more cleanly cut. The chilled
water slurry circulation loop extends from the underwater
pelletizer, carrying the pellet-water slurry to a pellet drying
unit, and then recycles the water back to the underwater
pelletizer.
[0070] In an embodiment, the residence time of the pellets in the
chilled water slurry circulation loop is at least 10 seconds, or at
least 20 seconds, or at least 30 seconds, or preferably at least 40
seconds, or at least 50 seconds or more. As fresh pellets tend to
bridge and agglomerate if the pellets have not had adequate time to
crystallize and harden, or if the polymer is a low crystallinity
polymer, it is preferred that the pellets have sufficient residence
time in the pellet water loop.
[0071] In another embodiment chilled water removes the pellets from
the cutter blade and transports them through a screen which catches
and removes coarsely aggregated or agglomerated pellets. The water
then transports the pellets through a dewatering device and into a
centrifugal dryer or fluidized bed to remove excess surface
moisture from the pellets. The pellets may then pass through a
discharge chute for collection or may proceed to additional
processing including which can include pellet coating,
crystallization, or further cooling as required to achieve the
desired product.
[0072] The pelletizing die can be used to make pellets in shapes
not limited to spheres, rods, slats, or polygons. Preferably, near
spherical pellets are made. A pellet shape that will allow the
pellets to easily flow is preferred.
[0073] The speed at which the pelletizer operates is selected
according to the die plate size, number of orifices in the die, and
to achieve the desired pellet size and shape. The number of
orifices in the die and the orifice geometry are selected as
appropriate for the polymer feed flow rate and melt material as is
known to those skilled in the art.
[0074] Optionally, an antiblocking agent may be added to the water
in the underwater pelletizing water box or chilled water slurry
loop. The addition of an antiblock to the pellet water loop is
useful to prevent pellets from sticking together in the loop and
plugging the lump catcher screen upstream of the dryer.
[0075] The temperature of the water, the rotation rate of the
cutter blades, and the flow rate of the polymer melt through the
pelletizing die all contribute to the production of proper pellet
geometries. Additionally, the temperature of the pellets, both in
the interior and the exterior, also influence the formation of the
pellets as well as the drying of the pellets.
[0076] Incomplete crystallization of the polymer material in the
pellets after the pellets have exited the pellet-water slurry loop
can lead to poor pellet geometry, pellet deformation, and reduced
ability of the pellets to freely flow. The degree of
crystallization of the pellets is affected by residence time and
temperature of the pellets. Additionally, the pellet hardness
varies with residence time and temperature.
Drying
[0077] In an embodiment of the invention, the pellets are dried
after exiting the underwater pelletizing unit. Drying can be by any
process, including centrifuge, fluid bed drier in which a heated
gas (e.g., air) is passed through a fluidized bed of the pellets,
or a flash dryer. Preferably, the pellets are dried in a
centrifugal dryer, which is connected to the outlet of the
underwater pelletizing die. Examples of useful centrifugal driers
are those available from Gala Industries, such as those disclosed
in U.S. Pat. Nos. 6,807,748 B2; 7,024,794 B1; and 7,171,762 B2, all
incorporated herein by reference.
[0078] In one embodiment, the pellet-water slurry passes through an
agglomerate catcher which may comprise a round wire grid or coarse
screen to remove oversize chunks or agglomerates of pellets. The
pellet-water slurry may then optionally pass through a dewatering
device, or a series of dewatering devices, containing baffles and
an angular feed screen which collectively reduce the water content,
preferably 90% or more, or 98% or more. The removed water may then
pass through a fines removal screen into a water tank/reservoir so
that it may be recycled or disposed. The pellets may then pass
through a centrifugal dryer to remove any remaining water. The
dried pellets then exit the centrifugal dryer and proceed to
storage or may be further processed with coatings, additional
crystallization or further cooled as is well understood by those
skilled in the art.
[0079] In another embodiment, after the pellets exit the
centrifugal dryer they proceed to a further drying step to
eliminate any excess moisture. The further drying step may be a
fluid bed dryer or another means of drying known to those of
ordinary skill in the art.
[0080] Desirably the pellets are dry when they are packaged. The
pellets are considered to be dry when they comprise less than 1 wt
% moisture, or less than 0.5 wt % moisture, or less than 0.1 wt %
moisture, or most preferably less than 0.08 wt % moisture. It may
be necessary to warm the pellets before packaging so that the cold
pellets will not collect condensation from atmospheric moisture.
The warming and drying step and the crystallization step may occur
at the same time in the same piece of equipment.
Additional Processing
[0081] After drying the pellets may be collected and batched or,
alternatively, may proceed for additional processing such as
further cooling or dusting/coating.
[0082] In one embodiment, the pellets are dusted/coated with an
external antiblock. An external antiblock can be used to allow for
easy flow of pellets through packaging equipment and to prevent
agglomeration in the final package. Any antiblock known to be
compatible with the polymer pellet may be used. Preferably the
pellets are dusted with the antiblock by mechanical mixing, so that
a consistent even coating of antiblock is formed on the pellet
surface. Mechanical mixing of the pellets and antiblock allows for
good antiblock coverage on the pellets and good adhesion/embedding
of antiblock particles on the pellets.
End Uses
[0083] Polymers used in this invention may be useful as adhesives,
viscosity modifiers, meltblown or spunbond non-wovens, packaging
HMAs, or polypropylene blending additives.
[0084] The adhesives of this invention can be used in any adhesive
application, including but not limited to, disposables, packaging,
laminates, pressure sensitive adhesives, tapes, labels, wood
binding, paper binding, non-wovens, road marking, reflective
coatings, and the like. The adhesives described above may be
applied to any substrate. In a particular embodiment, the adhesives
of this invention can be used in a packaging article.
[0085] An embodiment of the invention will now be more particularly
described with reference to the figures. FIG. 1 is a schematic
illustration of a conventional apparatus for pelletizing a polymer
feed, wherein the apparatus comprises a melt cooler, an underwater
pelletizer, and a drying apparatus. Referring to FIG. 1, the molten
polymer feed travels from a storage tank (not shown) or other
polymer feed source (not shown) through conduit 10 and enters the
melt cooler 11 at the melt cooler inlet 12. The polymer feed is
cooled as it is moves through the melt cooler, moving from the melt
cooler inlet 12 to the melt cooler outlet 13. Cooling medium flows
through a cooling jacket (not shown) around the melt cooler 11
flowing from the cooling medium inlet 14 to the cooling medium
outlet 15.
[0086] The cooled polymer feed exits the melt cooler 11 through the
melt cooler outlet 13 and travels through conduit 16 into the
underwater pelletizer 17. Optionally, the polymer feed may travel
through diverter valve 18 before entering the underwater pelletizer
17. The diverter valve 18 can be used to divert the polymer feed
from the cooling/pelletizing processing line to be recirculated or
purged/discharged from the apparatus. This can be particularly
useful when cleaning the cooling/pelletizing processing line.
Conduit 16 may be long or short. Alternatively, there is no conduit
16 and the polymer feed travels directly from the melt cooler
outlet 13 into the diverter valve or into the underwater pelletizer
17.
[0087] The underwater pelletizer 17 cuts the cooled polymer feed to
form pellets. The pellets then travel in a pellet-water slurry from
the underwater pelletizer 17 through conduit 19 into catch screen
20. Catch screen 20 can be used to collect agglomerated pellets.
The pellet-water slurry then travels through conduit 21 into the
centrifugal drier 22, where the pellets are separated from the
water and dried. In an alternate embodiment, there is no catch
screen 20 or conduit 21 and the pellet-water slurry travels
directly from the underwater pelletizer 17 through conduit 19
directly into the centrifugal drier 22.
[0088] The dried pellets then exit the centrifugal drier 22 through
conduit 23, where they can proceed for further processing or be
collected and packaged. The water separated from the pellets in the
centrifugal drier 22 can then travel through conduit 24 into water
storage tank 25, to be recycled back into the underwater pelletizer
17.
[0089] The water in the underwater pelletizer 17 is supplied from
water storage tank 25. Water flows from the storage tank 25 through
conduit 26 into a water cooler 27. Then the cooled water travels
through conduit 28 into the underwater pelletizer 17.
Alternatively, there is no water cooler 27 and water flows directly
from the storage tank 25 through conduit 26 into the underwater
pelletizer 17. Optionally, anti-block additives may be added into
the water in the water storage tank 25 through conduit 29.
[0090] In an embodiment, not shown, water from the storage tank 25
can travel through conduit 26 into a water cooler 27, and through a
conduit (not shown) into the melt cooler's 11 cooling jacket to act
as the cooling medium. The cooled water can enter the cooling
jacket through the cooling medium inlet 14 and exit the cooling
jacket through the cooling medium outlet 15, where it can then be
recycled back to the water storage tank 25 to be re-cooled.
[0091] FIG. 2 is a schematic illustration of an embodiment of the
inventive apparatus for pelletizing a polymer feed, wherein the
apparatus comprises a melt cooler, a cooling extruder, an
underwater pelletizer, and a drying apparatus. Some equipment in
FIG. 2 is similar to that in FIG. 1, e.g., the melt cooler, the
underwater pelletizer, and the drying apparatus, and as such have
been described using the same identifying numerals. Referring to
FIG. 2, the molten polymer feed travels from a storage tank (not
shown) or other polymer feed source (not shown) through conduit 10
and enters the melt cooler 11 at the melt cooler inlet 12. The
polymer feed is cooled as it is moves through the melt cooler,
moving from the melt cooler inlet 12 to the melt cooler outlet 13.
Cooling medium flows through a cooling jacket (not shown) around
the melt cooler 11 flowing from the cooling medium inlet 14 to the
cooling medium outlet 15.
[0092] The cooled polymer feed exits the melt cooler 11 through the
melt cooler outlet 13 and travels through conduit 16 into the
cooling extruder 30. Conduit 16 may be long or short. Alternatively
there is no conduit 16 and the polymer feed travels directly from
the melt cooler outlet 13 into the cooling extruder 30. The cooling
extruder 30 cools the polymer feed as the polymer feed moves from
the cooling extruder inlet along the length of the cooling extruder
barrel and out the cooling extruder outlet. The cooling extruder 30
may be a twin screw extruder.
[0093] The cooled extruded polymer feed then exits the cooling
extruder 30 and enters the underwater pelletizer 17. Optionally,
the polymer feed may travel through diverter valve 18 before
entering the underwater pelletizer 17. The diverter valve can be
used to divert the polymer feed from the cooling/pelletizing
processing line to be recirculated or purged/discharged from the
apparatus. This can be particularly useful when cleaning the
cooling/pelletizing processing line.
[0094] The underwater pelletizer 17 cuts the cooled extruded
polymer feed to form pellets. The pellets then travel in a
pellet-water slurry from the underwater pelletizer 17 through
conduit 19 into catch screen 20. Catch screen 20 can be used to
collect agglomerated pellets. The pellet-water slurry then travels
through conduit 21 into the centrifugal drier 22, where the pellets
are separated from the water and dried. In an alternate embodiment,
there is no catch screen 20 or conduit 21 and the pellet-water
slurry travels directly from the underwater pelletizer 17 through
conduit 19 directly into the centrifugal drier 22.
[0095] The dried pellets then exit the centrifugal drier 22 through
conduit 23, where they can proceed for further processing or be
collected and packaged. The water separated from the pellets in the
centrifugal drier 22 can then travel through conduit 24 into water
storage tank 25, to be recycled back into the underwater pelletizer
17.
[0096] The water in the underwater pelletizer 17 is supplied from
water storage tank 25. Water flows from the storage tank 25 through
conduit 26 into a water cooler 27. Then the cooled water travels
through conduit 28 into the underwater pelletizer 17.
Alternatively, there is no water cooler 27 and water flows directly
from the storage tank 25 through conduit 26 into the underwater
pelletizer 17. Optionally, anti-block additives may be added into
the water in the water storage tank 25 through conduit 29.
[0097] In an embodiment, not shown, water from the storage tank 25
can travel through conduit 26 into a water cooler 27, and through a
conduit (not shown) into the melt cooler's 11 cooling jacket to act
as the cooling medium. The cooled water would enter the cooling
jacket through the cooling medium inlet 14 and exit the cooling
jacket through the cooling medium outlet 15, where it can then be
recycled back to the water storage tank 25 to be re-cooled.
[0098] FIG. 3 is a schematic illustration of an apparatus for
pelletizing a polymer feed composed of an extruder, an underwater
pelletizer, and a drying apparatus. This embodiment is similar to
the embodiment shown in FIG. 1, except that the melt cooler 11 is
replaced with only an extruder.
[0099] While the illustrative embodiments have been described with
particularity, it will be understood that various other
modifications will be apparent to and can be readily made by those
skilled in the art without departing from the spirit and scope of
the invention. To the extent that this description is specific, it
is solely for the purposes of illustrating certain embodiments of
the invention and should not be taken as limiting the present
inventive concepts to these specific embodiments. Accordingly, it
in not intended that the scope of the claims appended hereto be
limited to the examples and descriptions set forth herein but
rather that the claims should be construed as encompassing all the
features of patentable novelty which reside in the present
invention, including all features which would be treated as
equivalents thereof by those skilled in the art to which the
invention pertains.
EXAMPLES
[0100] The method and apparatus for pelletizing a polymer feed will
now be further described with reference to the following
non-limiting examples.
[0101] In Examples 1-3 the polymer melt was composed of a hot melt
adhesive (HMA). The HMA comprised 86.1 wt % of a metallocene
catalyzed mixed-tacticity polypropylene polymer having a DSC heat
of fusion of from about 30 J/g to about 40 J/g and a melting
temperature of from about 130.degree. C. to about 135.degree. C.;
7.0 wt % of PARAFLINT.RTM. C80, commercially available from
Schumann Sasol, Ltd.; 3.5 wt % of ESCOREZ.RTM. 5300, commercially
available from ExxonMobil Chemical Company in Baytown, Tex.; 2.0 wt
% of MAPP 40, commercially available from Chusei of the USA; and
1.4 wt % of an anti-oxidant.
[0102] The HMA has a melting temperature by DSC of 130-135.degree.
C.; an onset of crystallization of 90-100.degree. C. as measured by
DSC; a viscosity at 177.degree. C. of 800-900 cP; a viscosity at
160.degree. C. of 1300-1400 cP; a Shore A hardness of 80-85; and a
softening point of 135-140.degree. C. The difference between the
HMA's melting temperature of from about 130.degree. C. to about
135.degree. C. and the HMA's crystallization temperature of from
about 90.degree. C. to about 100.degree. C. is due to a delayed
crystallization of the HMA at the DSC method prescribed cooling
rate.
[0103] As the HMA melt approaches the melting temperature
crystallization begins to form and the viscosity curve rises
steeply. This corresponds in a change from a clear melt to a cloudy
one. Further cooling of the melt completes the transition to an
opaque solid. The HMA's viscosity curve rises sharply near the
melting point where shear-induced crystallization begins to
occur.
Example 1
[0104] In this example, the polymer feed was pelletized using a
gear pump-melt cooler-pelletizer configuration. The
melt-cooler-pelletizer configuration was similar to that shown in
FIG. 1. A conventional gear pump was used to force the polymer feed
through the melt cooler and the underwater pelletizer. A
conventional melt cooler was used to cool the polymer feed and a
conventional underwater pelletizer was used to pelletize the
polymer feed. After the polymer feed was pelletized, the pellets
were dried in conventional centrifugal dryer.
[0105] Pelletizer run conditions for the five test runs of Example
1 can be found in Table 1. Test Nos. 1, 2, and 3 utilized a
standard 3-blade cutter. Test Nos. 4 and 5 used a 4-blade cutter.
In Test No. 1, no wrap-ups around the cutter assembly were
observed, however pellets slowly agglomerated at the dryer
discharge. In Test No. 2, the cutter current rose to 2 amps and
then polymer wrap-ups occurred around the pelletizer every three to
four minutes. In Test No. 3, new cutter blades were used, and no
wrap-ups were observed during 1.5 hours of operation. In Test No.
4, wrap-ups began to occur after several minutes of operation, and
the cutter current draw rose to 2 amps before the wrap-ups. In Test
No. 5, there were no wrap-ups observed during 1.5-2 hours of
operation.
[0106] During the underwater pelletizer operation, the cutter
blades were constantly being sharpened against the die plate. While
the blades stayed sharp, they become worn and were literally
shortened in length. This in turn affected the blade-die contact
and cutability of "extrudates" lacking strength, especially where
the viscosity of the extrudate was less than 1,200 cP at
190.degree. C. Thus, when pelletizing the extrudates with a
viscosity of less than 1,200 cP at 190.degree. C. cutter blades
with less than 80% wear were needed. This is demonstrated in a
comparison of Test No. 3 where new cutter blades were used and no
wrap-ups occurred, versus Test No. 2 where the blades were worn
from the prior Tests and wrap-ups frequently occurred.
[0107] The water temperature in the underwater pelletizer needed to
be as low as 33-34.degree. F. (about 1.degree. C.) in order to
pelletize the HMA products with a viscosity less than 800 cP. In
Test No. 4 warmer water was used and cutter wrap-ups quickly began
to occur. It is believed that the lower the water temperature, the
stronger the quenched extrudates are. However, one risk of using
low water temperatures is die-hole freeze off.
[0108] By extending the pellet residence time in water, e.g., by
extending the water-slurry piping length, the agglomeration of
softer pellets (e.g., pellets where the polymer crystallinity was
less than 30 J/g) in the dryer discharge was reduced.
[0109] Despite improved performance with colder pelletizer water,
and use of low wear cutter blades, the test runs in Example 1 were
often accompanied with slow crystallization on the melt cooler tube
walls. The accumulated crystallization on the melt cooler tube
walls resulted in a loss of heat transfer efficiency, which in turn
caused the melt temperature exiting the cooler to rise and the melt
viscosity through the die holes to drop. With increasingly
stringent viscosity targets, e.g., polymer feeds with viscosities
less than 800 cP at 190.degree. C., loss of cooling performance
often translated to start of wrap-ups in the pelletizer. Thus, with
the cutter wrap-ups and the fouled melt cooler, pelletizing with
the gear pump-melt cooler-pelletizer configuration was
difficult.
TABLE-US-00001 TABLE 1 Test No: 1 2 3 4 5 Polymer Feed Melt 1300
1000 900 800 800 Viscosity at 190.degree. C. Polymer Feed .DELTA.H
(J/g) 26 28 28 36 36 % wax (C80 + MAPP) 0 + 0 10 + 0 10 + 0 7 + 2 7
+ 2 Melt feed rate (lb/hr) 26 30 30 30 26 Die hole size/no.
0.110''/2 0.110''/2 0.110''/2 0.110''/2 0.110''/1 Die temperature
(F.) 260 260 260 260 260 Die pressure drop (psi) 400 250 250 250
400 Melt temperature (F.) 251 260 272 271 271 Water temperature
(F.) 37-39 38-40 50 50 33 Cutter rpm 3000 3000 3000 3000 3000
Cutter amps 0.6 0.6 (cutter 0.7 0.7 (cutter 0.7 currant draw
currant draw rose to 2 amps rose to 2 amps before wrap-up) before
wrap-up)
Example 2
[0110] In this example an apparatus similar to that of Example 1
was used, except that a larger multi-tube melt cooler exchanger was
used that had a higher heat transfer fluid temperature. A
conventional gear pump was used to force the polymer feed through
the melt cooler and the underwater pelletizer. A multi-tube melt
cooler was used to cool the polymer feed and a conventional
underwater pelletizer was used to pelletize the polymer feed. After
the polymer feed was pelletized, the pellets were dried in
conventional centrifugal dryer.
[0111] Pelletizer run conditions for the four test runs of Example
2 are found in Table 2. In Tests No. 1, 2, and 3, a shell and tube
heat exchanger with 13 tubes (having 0.5'' static mixer elements
inside) was used. In Test No. 4, the same melt cooler was used as
for Tests Nos. 1-3; however, the outer 6 of the 13 tubes were
plugged.
[0112] In Test Nos. 1 and 2 the temperature of the adhesive coming
out of the melt cooler slowly rose during the pelletizing run. In
Test No. 1 the pelletizer ran for 2 hours and then cutter wrap-ups
formed and the pelletizer could not be restarted. In Test No. 2 the
pelletizer ran for 2 hours with two trouble-free re-starts during
the 2 hour period, but after the 2 hour period cutter wrap-ups
formed and the pelletizer performance could not be repeated. In
Test No. 3, the pelletizer ran for 3.5 hours with no cutter
wrap-ups. In Test No. 4, the pelletizer ran for 1.5 hours with one
trouble-free restart during the 1.5 hour period, but after the 1.5
hour period the pelletizer performance could not be repeated.
[0113] In Example 2, the optimum melt feed rate was 25-27
lb/hr/hole. When the melt feed rate was increased to 30 lb/hr/hole,
as in Test No. 4, cutter wrap-ups occurred. Additionally, cutter
rpm had to be maintained at a high rate. When the cutter speed was
reduced from 3000 rpm to 750 rpm in Test 4 this led to cutter
wrap-ups. The use of a continuous hole profile and the 750 cutter
appeared to improve cutting performance, as compared to using
standard 450 blades in Test No. 1 where cutter-wrap ups were
observed.
[0114] However, even with the larger melt cooler used in Example 2
as compared to Example 1, the build up of crystallized polymer on
the tube walls was not prevented. Even at apparent optimum
conditions, the pelletizer was unable to handle polymer melts with
lower viscosities and in order to prevent partial crystallization
on the melt cooler tubes the melt cooler was had to operate at a
higher temperature.
TABLE-US-00002 TABLE 2 Test No.: 1 2 3 4 Melt Feed rate 50 158 30
100 (lb/hr) Die hole 0.125''/2 0.110''/4 0.110''/1 0.110''/4
size/no. Die design Standard Continuous Continuous Continuous land.
hole. hole. hole. Cutter type/no. Standard 75.degree./6
75.degree./8 75.degree./6 of blades (45.degree.)/4 Die 300 275 300
280 temperature (F.) Die pressure 300 275 300 280 drop (psi) Melt
230 .fwdarw. 241 258 .fwdarw. 269 268 250 temperature (F.) Melt
cooler oil 226/227 248/250 260/261 243/241 temperature in/ out (F.)
Water 40 35 35 37 temperature (F.) Cutter rpm 1000 2000 3000
3000
Example 3
[0115] In Example 3 adhesive pellets were made by cooling the
polymer feed in a cooling extruder and using the extruder's
pressure to drive the feed through the pelletizer. In this example
the extruder used was the Coperion ZSK-25 twin-screw extruder which
is commercially available from Coperion Corporation of Ramsey, N.J.
A conventional underwater pelletizer and drying apparatus were
used. Test run conditions for the two test runs of Example 3 are
found in Table 3.
[0116] In Test No. 1, adhesive pellets were fed to the twin-screw
extruder, melted, and cooled to 106.degree. C. to form a paste to
be pelletized. During two days of operation no wrap-ups or pellet
quality issues were encountered.
[0117] In Test No. 2 an adhesive melt was fed to the extruder,
cooled, and then pelletized. Additionally, in Test No. 2, the screw
flights on the extruder screw were arranged with wide flights
directly under the melt feed port (extruder inlet) and the flights
transitioned to close flights by the discharge zone (extruder
outlet) by the pelletizing die. For pelletizing the melt fed
extruder, the demonstrated heat transfer coefficient for barrel
cooling was calculated to be 31 Btu/hr-F-ft.sup.2.
[0118] The pellets produced in Example 3 were homogeneous,
suggesting that the crystallized components were readily dispersed
under the shear forces of the extruder.
TABLE-US-00003 TABLE 3 Test No: 1 2 Feed type/temperature
Solids/ambient Melt/300.degree. F. Feed Rate (lb/hr) 80 80 Extruder
speed (rpm) 360 300 Extruder drive torque (%) 40 24 Extruder barrel
temperature 176 111 (F.) Die hole size/no 0.125''/2 0.110''/4 Die
design Standard land Standard land Cutter type/no. of blades
Standard (45.degree.)/4 Standard (45.degree.)/6 Die temperature
(F.) 280 280 Die pressure drop (psi) 710 350 Melt temperature (F.)
220 163 Water temperature (F.) 45 37 Cutter rpm 2000 3000
[0119] Comparing the pellets formed in Examples 1, 2, and 3, it
could be seen that in Examples 1 and 2 cooling by a melt cooler
alone, allowed some crystallization and phase separation to occur
with in the melt which in turn led to the production of non-uniform
and non-homogenous pellets. There was not sufficient dispersive
mixing of the polymer feed with the melt cooler alone to produce
uniform and homogeneous pellets. Additionally, as the polymer feed
crystallized within the melt cooler, crystallization formed on the
melt cooler walls leading to a loss of heat transfer and reduced
cooling of the polymer feed. This in turn led to polymer wrap ups
around the cutter assembly and poor pellet formation.
[0120] In Example 3 using a cooling extruder allowed for sufficient
dispersive mixing of the polymer feed to eliminate phase separation
of the blended materials in the polymer feed. The cooling extruder
caused rigorous mixing and propagation of the polymer feed
maximizing the dispersive homogeneity of the melt. This allowed for
the formation of uniform and homogenous pellets.
Example 4
[0121] In Example 4 adhesive pellets were made by cooling a molten
adhesive feed in a melt cooler followed by further cooling and
pressurization in a cooling extruder. Pressure for extrusion
through the die was provided by the cooling extruder. The cooling
extruder was a Maris 92 mm twin-screw extruder which is
commercially available from MARIS S.p.A. The pelletizer was a
standard underwater pelletizer commercially available from GALA
Industries, Inc. of Eagle Rock, Va.
[0122] Three hot melt adhesive polymer feeds were successfully
pelletized in large quantities. The adhesives melts were those
described in U.S. Patent Application Publication 2004/0138392 A1.
Properties of the hot melt adhesive polymer feeds used can be found
in Table 4.
[0123] As seen in Table 5, for HMA A and HMA B the temperature of
the polymer melt at the outlet of the melt cooler was significantly
below that of the softening point of the adhesive. The cooling
extruder further reduced the polymer melt temperature, thus the
polymer melt's temperature at the diverter valve following the
cooling extruder approached the crystallization temperature of the
HMA.
[0124] Thus, by precisely cooling the polymer melt in the cooling
extruder to a temperature that was less than the softening point of
the polymer melt but greater than the crystallization temperature,
the polymer feeds were able to be easily pelletized. Additionally,
the cooling extruder mixed and maintained the homogeneity of the
polymer feed allowing uniform pellets to be formed. Thus, by using
a cooling extruder the temperature of the polymer feed was able to
be precisely controlled to an optimum temperature where the feed
was easily pelletized yet uniformly dispersed.
TABLE-US-00004 TABLE 4 Softening Point Crystallization Viscosity
(cP @ Adhesive (C) Temp (C.) 177.degree. C.) HMA A 133 91 950 HMA B
132 66 950 HMA C 126 36 13,000
TABLE-US-00005 TABLE 5 Melt Cooler Melt Cooler Outlet Diverter
Inlet Pressure Die Face Pressure Adhesive Temp (C.) Temp (C.) (PSI)
(PSI) HMA A 115 103 84 719 HMA B 125 95 97 671 HMA C 134 103 343
797
* * * * *